AlmaLinux OS 9 must not respond to Internet Control Message Protocol (ICMP) echoes sent to a broadcast address.

Responding to broadcast (ICMP) echoes facilitates network mapping and provides a vector for amplification attacks.

Ignoring ICMP echo requests (pings) sent to broadcast or multicast addresses makes the system slightly more difficult to enumerate on the network.

Verify AlmaLinux OS 9 does not respond to ICMP echoes sent to a broadcast address with the following command:

$ sysctl net.ipv4.icmp_echo_ignore_broadcasts

net.ipv4.icmp_echo_ignore_broadcasts = 1

If the returned line does not have a value of "1", this is a finding.

Configure AlmaLinux OS 9 to use reverse path filtering on all IP interfaces.

Create a numbered *.conf file in /etc/sysctl.d/ with the following content:

net.ipv4.icmp_echo_ignore_broadcasts = 1

The system configuration files need to be reloaded for the changes to take effect. To reload the contents of the files, run the following command:

$ sysctl –system